A New Family of Perovskite Catalysts for Oxygen-Evolution Reaction in Alkaline Media: BaNiO3 and BaNi0.83O2.5

Structural evolution and catalytic mechanisms of perovskite oxides in electrocatalysis

Electrocatalysis plays a pivotal role in driving the progress of modern technologies and industrial processes such as energy conversion and emission reduction. Perovskite oxides, an important family of electrocatalysts, have garnered substantial attention in diverse catalytic reactions because of their highly tunable composition and structure, as well as their considerable activity and stability. This review delves into the mechanisms of electrocatalytic reactions that use perovskite oxides as electrocatalysts, while also providing a comprehensive summary of the potential key factors that influence catalytic activity across various reactions. Furthermore, this review offers an overview of advanced characterizations used for studying catalytic mechanisms and proposes approaches to designing highly efficient perovskite oxide electrocatalysts.

Electrocatalytic Properties of Quasi-2D Oxides LaSrMn0.5M0.5O4 (M = Co, Ni, Cu, and Zn) for Hydrogen and Oxygen Evolution Reactions

Designing cost-effective and highly efficient electrocatalysts for water splitting is a significant challenge. We have systematically investigated a series of quasi-2D oxides, LaSrMn0.5M0.5O4 (M = Co, Ni, Cu, Zn), to enhance the electrocatalytic properties of the two half-reactions of water-splitting, namely oxygen and hydrogen evolution reactions (OER and HER). The four materials are isostructural, as confirmed by Rietveld refinements with X-ray diffraction. The oxygen contents and metal valence states were determined by iodometric titrations and X-ray photoelectron spectroscopy. Electrical conductivity measurements in a wide range of temperatures revealed semiconducting behavior for all four materials. Electrocatalytic properties were studied for both half-reactions of water-splitting, namely, oxygen-evolution and hydrogen-evolution reactions (OER and HER). For the four materials, the trends in both OER and HER were the same, which also matched the trend in electrical conductivities. Among them, LaSrMn0.5Co0.5O4 showed the best bifunctional electrocatalytic activity for both OER and HER, which may be attributed to its higher electrical conductivity and favorable electron configuration.

Perovskite Oxides Toward Oxygen Evolution Reaction: Intellectual Design Strategies, Properties and Perspectives

Developing electrochemical energy storage and conversion devices (e.g., water splitting, regenerative fuel cells and rechargeable metal-air batteries) driven by intermittent renewable energy sources holds a great potential to facilitate global energy transition and alleviate the associated environmental issues. However, the involved kinetically sluggish oxygen evolution reaction (OER) severely limits the entire reaction efficiency, thus designing high-performance materials toward efficient OER is of prime significance to remove this obstacle. Among various materials, cost-effective perovskite oxides have drawn particular attention due to their desirable catalytic activity, excellent stability and large reserves. To date, substantial efforts have been dedicated with varying degrees of success to promoting OER on perovskite oxides, which have generated multiple reviews from various perspectives, e.g., electronic structure modulation and heteroatom doping and various applications. Nonetheless, the reviews that comprehensively and systematically focus on the latest intellectual design strategies of perovskite oxides toward efficient OER are quite limited. To bridge the gap, this review thus emphatically concentrates on this very topic with broader coverages, more comparative discussions and deeper insights into the synthetic modulation, doping, surface engineering, structure mutation and hybrids. More specifically, this review elucidates, in details, the underlying causality between the being-tuned physiochemical properties [e.g., electronic structure, metal-oxygen (M-O) bonding configuration, adsorption capacity of oxygenated species and electrical conductivity] of the intellectually designed perovskite oxides and the resulting OER performances, coupled with perspectives and potential challenges on future research. It is our sincere hope for this review to provide the scientific community with more insights for developing advanced perovskite oxides with high OER catalytic efficiency and further stimulate more exciting applications.

Graphical Abstract

Improving the Oxygen Evolution Reaction: Exsolved Cobalt Nanoparticles on Titanate Perovskite Catalyst

Perovskites are an important class of oxygen evolution reaction (OER) catalysts due to highly tunable compositions and adaptable characteristics. However, perovskite-based catalysts can have limited atom utilization efficiency due to large particle size, resulting in low mass activity. Herein, Cobalt nanoparticles are exsolved from La0.2+2x Ca0.7-2x Ti1-x Cox O3 perovskite and applied in OER. Upon reduction in the 5% H2 /N2 atmosphere at 800 °C for 2 h, the Co exsolved perovskite catalyst (R-LCTCo0.11) exhibits optimal OER performance. The mass activity of R-LCTCo0.11 reaches ≈1700 mA mg-1 at an overpotential of 450 mV, which is 17 times and 3 times higher than that of LCTCo0.11 (97 mA mg-1 ) and R-Mix (560 mA mg-1 ) catalysts respectively, surpassing the benchmark catalyst RuO2 (42.7 mA mg-1 of oxide at η = 470 mV). Electrochemical impedance spectroscopy (EIS) data reveals that R-LCTCo0.11 has the lowest charge transfer resistance (Rct  = 58 Ω), demonstrating the highest catalytic and kinetic activity for OER. Furthermore, this catalyst shows high stability during an accelerated durability test of 10 h electrolysis and 1000 cycles cyclic voltammetry (CV). This work demonstrates that nanoparticle exsolution from a doped perovskite is an effective strategy for improving the atom utilization efficiency in OER.

A Mechanistic Overview of the Current Status and Future Challenges in Air Cathode for Aluminum Air Batteries

Aluminum air batteries (AABs) are a desirable option for portable electronic devices and electric vehicles (EVs) due to their high theoretical energy density (8100 Wh K-1 ), low cost, and high safety compared to state-of-the-art lithium-ion batteries (LIBs). However, numerous unresolved technological and scientific issues are preventing AABs from expanding further. One of the key issues is the catalytic reaction kinetics of the air cathode as the fuel (oxygen) for AAB is reduced there. Additionally, the performance and price of an AAB are directly influenced by an air electrode integrated with an oxygen electrocatalyst, which is thought to be the most crucial element. In this study, we covered the oxygen chemistry of the air cathode as well as a brief discussion of the mechanistic insights of active catalysts and how they catalyze and enhance oxygen chemistry reactions. There is also extensive discussion of research into electrocatalytic materials that outperform Pt/C such as nonprecious metal catalysts, metal oxide, perovskites, metal-organic framework, carbonaceous materials, and their composites. Finally, we provide an overview of the present state, and possible future direction for air cathodes in AABs.

Enhancement of Electrocatalytic and Pseudocapacitive Properties as a Function of Structural Order in A2Fe2O5 (A = Sr, Ba)

Significant enhancements of electrocatalytic activities for both half-reactions of water-electrolysis, i.e., oxygen evolution reaction (OER) and hydrogen evolution reaction (HER), as well as pseudocapacitive charge-storage properties are demonstrated upon changing the structural order in a perovskite-type system. The structural change is prompted by the increase in the ionic radius of the A-site ion in A2Fe2O5. The structure of Sr2Fe2O5 consists of alternating layers of FeO6 octahedra and FeO4 tetrahedra, whereas Ba2Fe2O5 comprises seven different coordination geometries for Fe. We note that the catalytically active metal, i.e., iron, and the oxygen stoichiometry are the same for both materials. Nevertheless, the change in the structural order results in significantly greater electrocatalytic activity of Ba2Fe2O5, manifested in smaller overpotentials, smaller charge-transfer resistance, greater electrocatalytic current, and faster reaction kinetics. In addition, this material shows significantly enhanced pseudocapacitive properties, with greater specific capacitance and energy density compared to Sr2Fe2O5. These findings indicate the important role of structural order in directing the electrochemical properties.

Designing Oxide Catalysts for Oxygen Electrocatalysis: Insights from Mechanism to Application

The electrochemical oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are fundamental processes in a range of energy conversion devices such as fuel cells and metal-air batteries. ORR and OER both have significant activation barriers, which severely limit the overall performance of energy conversion devices that utilize ORR/OER. Meanwhile, ORR is another very important electrochemical reaction involving oxygen that has been widely investigated. ORR occurs in aqueous solutions via two pathways: the direct 4-electron reduction or 2-electron reduction pathways from O2 to water (H2O) or from O2 to hydrogen peroxide (H2O2). Noble metal electrocatalysts are often used to catalyze OER and ORR, despite the fact that noble metal electrocatalysts have certain intrinsic limitations, such as low storage. Thus, it is urgent to develop more active and stable low-cost electrocatalysts, especially for severe environments (e.g., acidic media). Theoretically, an ideal oxygen electrocatalyst should provide adequate binding to oxygen species. Transition metals not belonging to the platinum group metal-based oxides are a low-cost substance that could give a d orbital for oxygen species binding. As a result, transition metal oxides are regarded as a substitute for typical precious metal oxygen electrocatalysts. However, the development of oxide catalysts for oxygen reduction and oxygen evolution reactions still faces significant challenges, e.g., catalytic activity, stability, cost, and reaction mechanism. We discuss the fundamental principles underlying the design of oxide catalysts, including the influence of crystal structure, and electronic structure on their performance. We also discuss the challenges associated with developing oxide catalysts and the potential strategies to overcome these challenges.

Use of A-Site Metal Exsolution from a Hydrated Perovskite Titanate for Combined Steam and CO2 Reforming of Methane

Metal segregation from a perovskite oxide (ABO₃) usually referring to "redox metal exsolution" has recently been used for in situ preparation of a well-designed catalyst where metal nanoparticles are homogeneously and strongly embedded on perovskite scaffolds upon reduction. The exsolution concept of B-site transition metal ions has grown, but several issues such as segregation of A-site alkaline-earth metal ions (altering electronic structures of the perovskite surface, causing deformation of perovskite structures, or creating undesirable products via side reactions) and carbon formations on metal nanoparticles should be addressed for stable catalysts in greenhouse gas (CO₂ or CH₄) conversion. Here, we suggest a new approach to designing metal-perovskite composite catalysts via A-site metal segregation from a hydrated perovskite titanate. In situ formation of A-site-deficient hydrated CaTiO₃ accompanied with Ni exsolution solids leads to ∼78 and 65% of CH₄ and CO₂ conversion, respectively, suppressing carbon formations and alkaline-earth metal segregations in combined steam and carbon dioxide reforming of methane at 700 °C. It would help to design active and stable metal-perovskite catalysts for energy and environmental applications.

Fullerene-Derived Porous and Defective N-Doped Carbon Nanosheets as Advanced Trifunctional Metal-Free Electrocatalysts

Dopants and defects are crucial for multifunctional carbon-based metal-free electrocatalysts, but the rational design and facile production remain as a big challenge. Herein, we report a novel strategy using salt-assisted pyrolysis of derivatized fullerenes to fabricate defect-rich and N-doped carbon nanosheets. Molecular level modification of C₆₀ via amination and hydroxylation gives rise to well-defined fullerol molecules bearing N-containing groups (FNG). Subsequent calcination of FNG and NaCl at 750 °C generates porous carbon nanosheets (FNCNs-750) and turns the N-containing groups into high-level N dopants (12.43 at.%). Further pyrolysis of FNCNs-750 at 900 °C (FNCNs-900) leads to a reduced N content populated by graphitic-N. Meanwhile, abundant intrinsic defects (e. g., topological defects and edges) are created due to the breakdown of fullerene cages and partial removal of edged N atoms. These structural features endow FNCNs-900 with outstanding trifunctional catalytic performance, better than or close to the noble metal-based benchmark catalysts.

Enhancement on PrBa0.5Sr0.5Co1.5Fe0.5O5 Electrocatalyst Performance in the Application of Zn-Air Battery

Due to the insufficient stability and expensive price of commercial precious metal catalysts like Pt/C and IrO2, it is critical to study efficiently, stable oxygen reduction reaction as well as oxygen evolution reaction (ORR/OER) electrocatalysts of rechargeable Zn-air batteries. PrBa0.5Sr0.5Co1.5Fe0.5O5 (PBSCF) double perovskite was adopted due to its flexible electronic structure as well as higher electro catalytic activity. In this study, PBSCF was prepared by the citrate-EDTA method and the optimized amount of PBSCF-Pt/C composite was used as a potential ORR/OER bifunctional electrocatalyst in 0.1 M KOH. The optimized composite exhibited excellent OER intrinsic activity with an onset potential of 1.6 V and Tafel slope of 76 mV/dec under O2-saturated 0.1 M KOH. It also exhibited relatively competitive ORR activity with an onset potential of 0.9 V and half-wave potential of 0.78 V. Additionally, Zn–air battery with PBSCF composite catalyst showed relatively good stability. All these results illustrate that PBSCF-Pt/C composite is a promising bifunctional electrocatalyst for rechargeable Zn-air batteries.

Water electrolysis: from textbook knowledge to the latest scientific strategies and industrial developments

Replacing fossil fuels with energy sources and carriers that are sustainable, environmentally benign, and affordable is amongst the most pressing challenges for future socio-economic development. To that goal, hydrogen is presumed to be the most promising energy carrier. Electrocatalytic water splitting, if driven by green electricity, would provide hydrogen with minimal CO2 footprint. The viability of water electrolysis still hinges on the availability of durable earth-abundant electrocatalyst materials and the overall process efficiency. This review spans from the fundamentals of electrocatalytically initiated water splitting to the very latest scientific findings from university and institutional research, also covering specifications and special features of the current industrial processes and those processes currently being tested in large-scale applications. Recently developed strategies are described for the optimisation and discovery of active and durable materials for electrodes that ever-increasingly harness first-principles calculations and machine learning. In addition, a technoeconomic analysis of water electrolysis is included that allows an assessment of the extent to which a large-scale implementation of water splitting can help to combat climate change. This review article is intended to cross-pollinate and strengthen efforts from fundamental understanding to technical implementation and to improve the 'junctions' between the field's physical chemists, materials scientists and engineers, as well as stimulate much-needed exchange among these groups on challenges encountered in the different domains.

Potassium Cobalt Pyrophosphate as a Nonprecious Bifunctional Electrocatalyst for Zinc-Air Batteries

Economic and sustainable (ecological) energy storage forms a major pillar of the global energy sector. Bifunctional electrocatalysts, based on oxygen electrolysis, play a key role in the development of rechargeable metal-air batteries. Pursuing precious metal-free economic catalysts, here, we report K₂CoP₂O₇ pyrophosphate as a robust cathode for secondary zinc-air batteries with efficient oxygen evolution and oxygen reduction (OER||ORR) activity. Prepared by autocombustion, nanoscale K₂CoP₂O₇ exhibited excellent oxygen reduction and evolution reactions among all phosphate-based electrocatalysts. In particular, the OER activity surpassed that of commercial RuO₂ with low overpotential (0.27 V). First-principles calculations revealed that the bifunctional activity is rooted in the Co active site with the CoO₅ local coordination in the most stable (110) surface. This nanostructured (tetragonal) pyrophosphate can be harnessed as an economic bifunctional catalyst for zinc-air batteries.

Highly active and stable surface structure for oxygen evolution reaction originating from balanced dissolution and strong connectivity in BaIrO3 solid solutions

Catalysts for the oxygen evolution reaction (OER) are receiving great interest since OER remains the bottleneck of water electrolyzers for hydrogen production. Especially, OER in acidic solutions is crucial since it produces high current densities and avoids precipitation of carbonates. However, even the acid stable iridates undergo severe dissolution during the OER. BaIrO₃ has the strongest IrO₆ connectivity and stable surface structure, yet it suffers from lattice collapse after OER cycling, making it difficult to improve the OER durability. In the present study, we have successfully developed an OER catalyst with both high intrinsic activity and stability under acidic conditions by preventing the lattice collapse after repeated OER cycling. Specifically, we find that the substitution of Ir-site with Mn for BaIrO₃ in combination with OER cycling leads to a remarkable activity enhancement by a factor of 28 and an overall improvement in stability. This dual enhancement of OER performance was accomplished by the novel strategy of slightly increasing the Ir-dissolution and balancing the elemental dissolution in BaIr_1−xMn_xO₃ to reconstruct a rigid surface with BaIrO₃-type structure. More importantly, the mass activity for BaIr_0.8Mn_0.2O₃ reached ∼73 times of that for IrO₂, making it a sustainable and promising OER catalyst for energy conversion technologies.

We have prevented lattice collapse and developed an OER catalyst with both high activity and stability by slightly increasing Ir-dissolution and balancing the elemental dissolution in BaIr_1−xMn_xO₃ for reconstructing the rigid catalytic surface.

Rational design of metal oxide catalysts for electrocatalytic water splitting

Electrocatalytic energy conversion between electricity and chemical bonding energy is realized through redox reactions with multiple charge transfer steps at the electrode-electrolyte interface. The surface atomic structure of the electrode materials, if appropriately designed, will provide an energetically affordable pathway with individual reaction intermediates that not only reduce the thermodynamic energy barrier but also allow an acceptably fast kinetic rate of the overall redox reaction. As one of the most abundant and stable forms, oxides of transitional metals demonstrated promising electrocatalytic activities towards multiple important chemical reactions. In this topical review, we attempt to discuss the possible avenues to construct the electrocatalytic active surface for this important class of materials for two essential chemical reactions for water splitting. A general introduction of the electrochemical water splitting process on the electrocatalyst surface with applied potential will be provided, followed by a discussion on the fundamental charge transfers and the mechanism. As the generally perceived active sites are chemical reaction dependent, we offer a general overview of the possible approaches to construct or create electrocatalytically active sites in the context of surface atomic structure engineering. The review concludes with perspectives that summarize challenges and opportunities in electrocatalysis and how these can be addressed to unlock the electrocatalytic potentials of the metal oxide materials.

Structural Variations of Metal Oxide-Based Electrocatalysts for Oxygen Evolution Reaction

Electrocatalytic oxygen evolution reaction (OER), an important electrode reaction in electrocatalytic and photoelectrochemical cells for a carbon-free energy cycle, has attracted considerable attention in the last few years. Metal oxides have been considered as good candidates for electrocatalytic OER because they can be easily synthesized and are relatively stable during the OER process. However, inevitable structural variations still occur to them due to the complex reaction steps and harsh working conditions of OER, thus impending the further insight into the catalytic mechanism and rational design of highly efficient electrocatalysts. The aim of this review is to disclose the current research progress toward the structural variations of metal oxide-based OER electrocatalysts. The origin of structural variations of metal oxides is discussed. Based on some typical oxides performing OER activity, the external and internal factors that influence the structural stability are summarized and then some general approaches to regulate the structural variation process are provided. Some operando methods are also concluded to monitor the structural variation processes and to identify the final active structure. Additionally, the unresolved problems and challenges are presented in an attempt to get further insight into the mechanism of structural variations and establish a rational structure-catalysis relationship.

Promoted lithium polysulfide conversion and immobilization by conductive titanium oxynitride-carbon architecture design for advanced lithium-sulfur batteries

In this work, a multifunctional oxygen deficient titanium oxynitride skeleton featuring a Co-metal-decorated three-dimensional ordered macroporous (3DOM) structure embedded with N-doped carbon nanotubes (Co@TiO_xN_y/N-CNTs) is fabricated as a sulfur host for lithium-sulfur (Li-S) batteries. The unique 3DOM framework provides abundant space for sulfur accommodation and effective pathways for electrolyte infiltration. The robust titanium oxynitride skeleton also ensures good structural integrity during the repeated charge/discharge cycling. Meanwhile, the introduction of oxygen defects not only improves the intrinsic conductivity of the TiO₂ skeleton but also enhances its capability for lithium polysulfide (LiPS) trapping. The N-CNTs embedded in the macroporous framework form an ultra-high conductive network and also provide rich micropores for sulfur distribution and physical confinement. The highly dispersed Co nanoparticles uniformly anchored on TiO_xN_y and N-CNTs act as electrocatalysts promoting the conversion of LiPSs. Attributed to these features, the Co@TiO_xN_y/N-CNTs/S electrode presents good rate capability and excellent cycling performance. Even under a sulfur loading of 6.34 mg cm^-2 and a low electrolyte to sulfur ratio (E/S = 8 μL mg^-1), a high area capacity of 5.05 mA h cm^-2 can be achieved after 50 cycles. The flexible pouch cell also delivers an impressive discharge capacity of 972 mA h g^-1 after 100 cycles under a sulfur loading of 4 mg cm^-2. This work offers a rational strategy for the design of advanced sulfur cathodes for Li-S batteries.

NaBH4-reduction induced tunable oxygen vacancies in LaNiO2.7 to enhance the oxygen evolution reaction

Tunable oxygen vacancies of LaNiO3 (LNO-Vo) are realized by theoretical prediction and the NaBH4-reduction approach. The LNO2.7 catalyst exhibits superior catalytic activity and long-term stability for water oxidation. Direct evidence of the active site center and the intermediates is observed from in situ Raman spectra and DFT calculations.

Atomic Ni and Cu co-anchored 3D nanoporous graphene as an efficient oxygen reduction electrocatalyst for zinc-air batteries

Highly active, cost-effective and durable electrocatalysts for the oxygen reduction reaction (ORR) are critically important for renewable energy conversion and storage. Here we report a 3D bicontinuous nitrogen doped nanoporous graphene electrocatalyst co-anchoring with atomically dispersed nickel and copper atoms ((Ni,Cu)-NG) as a highly active single-atom ORR catalyst, fabricated by the combination of chemical vapor deposition and high temperature gas transportation. The resultant (Ni,Cu)-NG exhibits an exceptional ORR activity in alkaline electrolytes, comparable to the Pt-based benchmarks, from the synergistic effect of the CuNx and NiNx complexes. Endowed with high catalytic activity and outstanding durability under harsh electrochemical environments, rechargeable zinc-air batteries using (Ni,Cu)-NG as the cathodes show excellent energy efficiency (voltage gap of 0.74 V), large power density (150.6 mW cm-2 at 250 mA cm-2) and high cycling stability (>500 discharge-charge cycles at 10 mA cm-2). This study may pave an efficient avenue for designing highly durable single-atom ORR catalysts for metal-air batteries.

In Situ Cation Intercalation in the Interlayer of Tungsten Sulfide with Overlaying Layered Double Hydroxide in a 2D Heterostructure for Facile Electrochemical Redox Activity

The role of electrochemical interfaces in energy conversion and storage is unprecedented and more so the interlayers of two-dimensional (2D) heterostructures, where the physicochemical nature of these interlayers can be adjusted by cation intercalation. We demonstrate in situ intercalation of Ni²⁺ and Co²⁺ with similar ionic radii of ∼0.07 nm in the interlayer of 1T-WS₂ while electrodepositing NiCo layered double hydroxide (NiCo-LDH) to create a 2D heterostructure. The extent of intercalation varies with the electrodeposition time. Electrodeposition for 90 s results in 22.4-nm-thick heterostructures, and charge transfer ensues from NiCo-LDH to 1T-WS₂, which stabilizes the higher oxidation states of Ni and Co. Density functional theory calculations validate the intercalation principle where the intercalated Ni and Co d electrons contribute to the density of states at the Fermi level of 1T-WS₂. Water electrolysis is taken as a representative redox process. The 90 s electrodeposited heterostructure needs the relatively lowest overpotentials of 134 ± 14 and 343 ± 4 mV for hydrogen and oxygen evolution reactions, respectively, to achieve a current density of ±10 mA/cm² along with exceptional durability for 60 h in 1 M potassium hydroxide. The electrochemical parameters are found to correlate with enhanced mass diffusion through the cation and Cl^--intercalated interlayer spacing of 1T-WS₂ and the number of active sites. While 1T-WS₂ is mostly celebrated as a HER catalyst in an acidic medium, with the help of intercalation chemistry, this work explores an unfound territory of this transition-metal dichalcogenide to catalyze both half-reactions of water electrolysis.

Electronic Structure Tuning of 2D Metal (Hydr)oxides Nanosheets for Electrocatalysis

2D metal (hydr)oxide nanosheets have captured increasing interest in electrocatalytic applications aroused by their high specific surface areas, enriched chemically active sites, tunable physiochemical properties, etc. In particular, the electrocatalytic reactivities of materials greatly rely on their surface electronic structures. Generally speaking, the electronic structures of catalysts can be well adjusted via controlling their morphologies, defects, and heterostructures. In this Review, the latest advances in 2D metal (hydr)oxide nanosheets are first reviewed, including the applications in electrocatalysis for the hydrogen evolution reaction, oxygen reduction reaction, and oxygen evolution reaction. Then, the electronic structure-property relationships of 2D metal (hydr)oxide nanosheets are discussed to draw a picture of enhancing the electrocatalysis performances through a series of electronic structure tuning strategies. Finally, perspectives on the current challenges and the trends for the future design of 2D metal (hydr)oxide electrocatalysts with prominent catalytic activity are outlined. It is expected that this Review can shed some light on the design of next generation electrocatalysts.

Spontaneous phase segregation of Sr2NiO3 and SrNi2O3 during SrNiO3 heteroepitaxy

Recent discovery of superconductivity in Nd0.8Sr0.2NiO2 motivates the synthesis of other nickelates for providing insights into the origin of high-temperature superconductivity. However, the synthesis of stoichiometric R 1-x Sr x NiO3 thin films over a range of x has proven challenging. Moreover, little is known about the structures and properties of the end member SrNiO3 Here, we show that spontaneous phase segregation occurs while depositing SrNiO3 thin films on perovskite oxide substrates by molecular beam epitaxy. Two coexisting oxygen-deficient Ruddlesden-Popper phases, Sr2NiO3 and SrNi2O3, are formed to balance the stoichiometry and stabilize the energetically preferred Ni2+ cation. Our study sheds light on an unusual oxide thin-film nucleation process driven by the instability in perovskite structured SrNiO3 and the tendency of transition metal cations to form their most stable valence (i.e., Ni2+ in this case). The resulting metastable reduced Ruddlesden-Popper structures offer a testbed for further studying emerging phenomena in nickel-based oxides.

Grain Boundaries Boost Oxygen Evolution Reaction in NiFe Electrocatalysts

In a polycrystalline material, the grain boundaries (GBs) can be effective active sites for catalytic reactions by providing an electrodynamically favorable surface. Previous studies have shown that grain boundary density is related to the catalytic activity of the carbon dioxide reduction reaction, but there is still no convincing evidence that the GBs provide surfaces with enhanced activity for oxygen evolution reaction (OER). Combination of various electrochemical measurements and chemical analysis reveals the GB density at surface of NiFe electrocatalysts directly affects the overall OER. In situ electrochemical microscopy vividly shows that the OER occurs mainly at the GB during overall reaction. It is observed that the reaction determining steps are altered by grain boundary densities and the meaningful work function difference between the inside of grain and GBs exists. High-resolution transmission electron microscopy shows that extremely high index planes are exposed at the GBs, enhancing the oxygen evolution activity. The specific nature of GBs and its effects on the OER demonstrated in this study can be applied to the various polycrystalline electrocatalysts.

Surface Tuning to Promote the Electrocatalysis for Oxygen Evolution Reaction: From Metal-Free to Cobalt-Based Carbon Electrocatalysts

Heterogeneous electrocatalytic reactions only occur at the interface between the electrocatalyst and reactant. Therefore, the active sites are only necessary to be distributed on the surface of the electrocatalyst. Based on this motivation, here, we demonstrate a systematic study on surface tuning for a carbon-based electrocatalyst from metal-free (with the heteroatoms N and S, NS/C) to metal-containing surfaces (with Co, N, and S, CoNS/C). The CoNS/C electrocatalyst was obtained by pyrolyzing the Co precoordinated and p-toluenesulfonate-doped polypyrrole (PPy). It was found that the coordination of Co on the PPy ring tuned the final carbon electrocatalyst into a catalyst with a CoN_x moiety-rich surface. In addition, the as-synthesized CoNS/C was determined to have a very high loading of cobalt up to 2.02 wt %. The pyrolysis of the cobalt-containing precursor tends to proceed toward a characteristic of a higher sp² carbon content, a higher surface area, and more nitrogen as well as active nitrogen sites than its metal-free counterpart. The most distinguished feature for such a catalyst is that the truly most active component is only distributed on the surface, in contrast with that of the conventional metal-N-based catalyst present throughout the bulky structure. Especially, the electrocatalytic activity toward oxygen evolution reaction (OER) has been investigated experimentally and theoretically. The results showed that the OER performance of the carbon-based electrocatalyst was remarkably boosted after the introduction of Co with an overpotential decrease from 678 to 345 mV at 10 mA cm^-2. Furthermore, CoNS/C displayed an excellent durability upon a long-term measurement. The apparent activation energy measurements revealed that the metal-rich surface contributed to overcome the energy barrier for OER. In addition, density functional theory calculations have been conducted to explain the correlated OER mechanism. This study is expected to provide an effective strategy for the design and the synthesis of highly active metal-nitrogen-type electrocatalysts with a high metal loading for various electrocatalytic reactions.

A highly efficient bifunctional electrocatalyst (ORR/OER) derived from GO functionalized with carbonyl, hydroxyl and epoxy groups for rechargeable zinc–air batteries

A highly efficient bifunctional electrocatalyst, Co–N/S/rGO, was prepared via modifying the surface functional groups of GO, and it showed good application prospects in zinc–air batteries.

Retention of anions in cobalt hydroxide with Ni substitution to emphasize the role of anions and cations for high current density in oxygen evolution reactions

Herein, we report the fabrication of remarkably fine nickel-substituted α-Co(OH)2 sheets using an ingenious co-precipitation method at a lower pH value. An α-CoNiOOH sheet retains the parent α-Co(OH)2 structure consisting of both tetrahedral (Td) and octahedral (Oh) sites with the retention of interlayer chloride ions, which is in contrast to the previous reports. The as-synthesized α-CoNiOOH sheet exhibits excellent oxygen evolution reactions (OERs) and produces a current of 10 mA cm-2 at an overpotential of merely 190 mV in an alkaline environment. Moreover, the α-CoNiOOH sheet attains an exceptionally high current density of 100 mA cm-2 at a low overpotential of only 270 mV. Additionally, this electrocatalyst possesses a 33 mV dec-1 Tafel slope with higher values of TOF (11 s-1) and double-layer capacitance (7.76 mF cm-2). This enhancement is attributed partially to the substitution of Ni during the conversion of α-Co(OH)2 to α-CoNiOOH and partially to the exceptionally thin sheets allowing potential octahedral sites for improved oxygen evolution reactions.

Atomic-Level Manipulations in Oxides and Alloys for Electrocatalysis of Oxygen Evolution and Reduction

As chemical reactions and charge-transfer simultaneously occur on the catalyst surface during electrocatalysis, numerous studies have been carried out to attain an in-depth understanding on the correlation among the surface structure and composition, the electrical transport, and the overall catalytic activity. Compared with other catalysis reactions, a relatively larger activation barrier for oxygen evolution/reduction reactions (OER/ORR), where multiple electron transfers are involved, is noted. Many works over the past decade thus have been focused on the atomic-scale control of the surface structure and the precise identification of surface composition change in catalyst materials to achieve better conversion efficiency. In particular, recent advances in various analytical tools have enabled noteworthy findings of unexpected catalytic features at atomic resolution, providing significant insights toward reducing the activation barriers and subsequently improving the catalytic performance. In addition to summarizing important surface issues, including lattice defects, related to the OER and ORR in this Review, we present the current status and discuss future perspectives of oxide- and alloy-based catalysts in terms of atomic-scale observation and manipulation.

CoNiFe Layered Double Hydroxide/RuO2.1 Nanosheet Superlattice as Carbon-Free Electrocatalysts for Water Splitting and Li-O2 Batteries

Efficient electrocatalysts are highly demanded for oxygen evolution reaction (OER) in water splitting and metal-air batteries. Here, superlattice structured materials composed of CoNiFe layered double hydroxide (LDH)/ruthenium oxide nanosheets are synthesized as carbon-free electrocatalysts for OER. The positively charged CoNiFe LDH and negatively charged RuO_2.1 are alternately stacked at the molecular level into superlattice-like hybrids by electrostatic interaction upon mixing their dispersions under suitable conditions. Such heterostructured composites are found to act as effective catalysts toward OER of water splitting with a small overpotential of 281 mV and Tafel plot of 48.9 mV/decade. Such composites also serve as efficient carbon-free cathode catalysts for aprotic Li-O₂ batteries with remarkable higher specific capacities and lower overvoltages than RuO₂ nanoparticles. The superior performance may be attributed to the peculiar superlattice structure, resulting in strong interfacial electronic coupling, better electrical conductivity, and the suppression of side reactions caused by traditional carbon-based materials. Furthermore, potential difference between RuO_2.1 and CoNiFe LDH nanosheets is observed directly by scanning Kelvin probe microscopy, indicating that electrostatic fields might be induced in the superlattice structures to benefit the transport of electrons and charged ions as well as the catalytic process.

Recent Advances of First d-Block Metal-Based Perovskite Oxide Electrocatalysts for Alkaline Water Splitting

First d-block metal-based perovskite oxides (FDMPOs) have garnered significant attention in research for their utilization in the water oxidation reaction due to their low cost, earth abundance, and promising activities. Recently, FDMPOs are being applied in electrocatalysis for the hydrogen evolution reaction (HER) and overall water splitting reaction. Numerous promising FDMPO-based water splitting electrocatalysts have been reported, along with new catalytic mechanisms. Therefore, an in-time summary of the current progress of FDMPO-based water splitting electrocatalysts is now considered imperative. However, few reviews have focused on this particular subject thus far. In this contribution, we review the most recent advances (mainly within the years 2014–2020) of FDMPO electrocatalysts for alkaline water splitting, which is widely considered to be the most promising next-generation technology for future large-scale hydrogen production. This review begins with an introduction describing the fundamentals of alkaline water electrolysis and perovskite oxides. We then carefully elaborate on the various design strategies used for the preparation of FDMPO electrocatalysts applied in the alkaline water splitting reaction, including defecting engineering, strain tuning, nanostructuring, and hybridization. Finally, we discuss the current advances of various FDMPO-based water splitting electrocatalysts, including those based on Co, Ni, Fe, Mn, and other first d-block metal-based catalysts. By conveying various methods, developments, perspectives, and challenges, this review will contribute toward the understanding and development of FDMPO electrocatalysts for alkaline water splitting.

Utilizing ion leaching effects for achieving high oxygen-evolving performance on hybrid nanocomposite with self-optimized behaviors

Ion leaching from pure-phase oxygen-evolving electrocatalysts generally exists, leading to the collapse and loss of catalyst crystalline matrix. Here, different from previous design methodologies of pure-phase perovskites, we introduce soluble BaCl₂ and SrCl₂ into perovskites through a self-assembly process aimed at simultaneously tuning dual cation/anion leaching effects and optimizing ion match in perovskites to protect the crystalline matrix. As a proof-of-concept, self-assembled hybrid Ba_0.35Sr_0.65Co_0.8Fe_0.2O_3-δ (BSCF) nanocomposite (with BaCl₂ and SrCl₂) exhibits the low overpotential of 260 mV at 10 mA cm^-2 in 0.1 M KOH. Multiple operando spectroscopic techniques reveal that the pre-leaching of soluble compounds lowers the difference of interfacial ion concentrations and thus endows the host phase in hybrid BSCF with abundant time and space to form stable edge/face-sharing surface structures. These self-optimized crystalline structures show stable lattice oxygen active sites and short reaction pathways between Co–Co/Fe metal active sites to trigger favorable adsorption of OH⁻ species.

Water oxidation catalysis may provide the electrons needed for sustainable fuel production, but catalysts often degrade under working conditions. Here, authors introduce soluble species into perovskites to exert positive ion leaching effects for enhancing perovskite stability and activity.

Spin Regulation on 2D Pd-Fe-Pt Nanomeshes Promotes Fuel Electrooxidations

Spin engineering provides a powerful strategy for manipulating the interaction between electrons in the d orbital and oxygen-containing adsorbates, while a little endeavor was performed to understand whether such a strategy can make a prosperous enhancement for fuel electrooxidations. Herein, we demonstrate that spin engineering of trimetallic Pd-Fe-Pt nanomeshes (NMs) can achieve superior enhancement for fuel electrooxidations. Magnetization characterizations reveal that Pd₅₉Fe₂₇Pt₁₄ NMs own the highest number of polarized spins (μ_b = 0.85 μB/f.u.), playing an important role on facilitating the adsorption of OH_ads to promote the oxidation of CO_ads, as confirmed by theoretical results. Consequently, the optimized Pd₅₉Fe₂₇Pt₁₄ NMs exhibit excellent methanol oxidation reaction activity and stability with a mass activity of 1.61 A mg_Pt^-1, 2.6-fold and 7.3-fold larger than those of PtRu/C and Pt/C. Such catalysts also present exceptional performances in ethanol oxidation and formic acid oxidation reactions. Our work highlights a new strategy for designing efficient electrocatalysts for fuel electrooxidations and beyond.

Enhancing the Electrocatalysis of LiNi0.5Co0.2Mn0.3O2 by Introducing Lithium Deficiency for Oxygen Evolution Reaction

LiNi_0.5Co_0.2Mn_0.3O₂ (NCM523), as a cathode material for rechargeable lithium-ion batteries, has attracted considerable attention and been successfully commercialized for decades. NCM is also a promising electrocatalyst for the oxygen evolution reaction (OER), and the catalytic activity is highly correlated to its structure. In this paper, we successfully obtain NCM523 with three different structures: spinel NCM synthesized at low temperature (LT-NCM), disordered NCM (DO-NCM) with lithium deficiency obtained at high temperature, and layered hexagonal NCM at high temperature (HT-NCM). By introducing lithium deficiency to tune the valence state of transition metals in NCM from Ni²⁺ to Ni³⁺, DO-NCM exhibits the best catalytic activity with the lowest onset potential (∼1.48 V) and Tafel slope (∼85.6 mV dec^-1), whereas HT-NCM exhibits the worst catalytic activity with the highest onset potential (∼1.63 V) and Tafel slope (∼241.8 mV dec^-1).

Interfacial Engineering of W2 N/WC Heterostructures Derived from Solid-State Synthesis: A Highly Efficient Trifunctional Electrocatalyst for ORR, OER, and HER

To meet the practical demand of overall water splitting and regenerative metal-air batteries, highly efficient, low-cost, and durable electrocatalysts for the oxygen reduction reaction (ORR), oxygen evolution reaction (OER), and hydrogen evolution reaction (HER) are required to displace noble metal catalysts. In this work, a facile solid-state synthesis strategy is developed to construct the interfacial engineering of W₂ N/WC heterostructures, in which abundant interfaces are formed. Under high temperature (800 °C), volatile CN_x species from dicyanodiamide are trapped by WO₃ nanorods, followed by simultaneous nitridation and carbonization, to form W₂ N/WC heterostructure catalysts. The resultant W₂ N/WC heterostructure catalysts exhibit an efficient and stable electrocatalytic performance toward the ORR, OER, and HER, including a half-wave potential of 0.81 V (ORR) and a low overpotential at 10 mA cm^-2 for the OER (320 mV) and HER (148.5 mV). Furthermore, a W₂ N/WC-based Zn-air battery shows outstanding high power density (172 mW cm^-2 ). Density functional theory and X-ray absorption fine structure analysis computations reveal that W₂ N/WC interfaces synergistically facilitate transport and separation of charge, thus accelerating the electrochemical ORR, OER, and HER. This work paves a novel avenue for constructing efficient and low-cost electrocatalysts for electrochemical energy devices.

Coordination environment evolution of Co(ii) during dehydration and re-crystallization processes of KCoPO4·H2O towards enhanced electrocatalytic oxygen evolution reaction

Development of efficient and stable electrodes for electrocatalytic oxygen evolution reaction (OER) is essential for energy storage and conversion applications, such as hydrogen generation from water splitting, rechargeable metal–air batteries and renewable fuel cells. Alkali metal cobalt phosphates show great potential as OER electrocatalysts. Herein, an original electrode design strategy is reported to realize an efficient OER electrocatalyst through engineering the coordination geometry of Co(ii) in KCoPO₄·H₂O by a facile dehydration process. Experimental result indicated that the dehydration treatment is accompanied by a structural transformation from orthorhombic KCoPO₄·H₂O to hexagonal KCoPO₄, involving a concomitant coordination geometry evolution of Co(ii) from octahedral to tetrahedral configuration. More significantly, the local structural evolution leads to an advantageous electronic effect, i.e. increased Co–O covalency, resulting in an enhanced intrinsic OER activity. To be specific, the as-produced KCoPO₄ can deliver a current density of 10 mA cm⁻² at a low overpotential of 319 mV with a small Tafel slope of 61.8 mV dec⁻¹ in alkaline electrolyte. Thus, this present research provides a new way of developing alkali metal transition-metal phosphates for efficient and stable electrocatalytic oxygen evolution reaction.

Coordination environment evolution of potassium cobalt phosphate towards enhanced electrocatalytic oxygen evolution reaction.

The hexagonal perovskite Ba0.5Sr0.5Co0.8Fe0.2O3−δ as an efficient electrocatalyst for the oxygen evolution reaction

Hexagonal structure perovskite Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ was sucessfully synthesized as an OER electrocatalyst, exhibiting excellent electrochemical performance in 0.1 M KOH.

Non-precious-metal catalysts for alkaline water electrolysis: operando characterizations, theoretical calculations, and recent advances

Advances of non-precious-metal catalysts for alkaline water electrolysis are reviewed, highlighting operando techniques and theoretical calculations in their development.

A review on fundamentals for designing oxygen evolution electrocatalysts

Electricity-driven water splitting can facilitate the storage of electrical energy in the form of hydrogen gas. As a half-reaction of electricity-driven water splitting, the oxygen evolution reaction (OER) is the major bottleneck due to the sluggish kinetics of this four-electron transfer reaction. Developing low-cost and robust OER catalysts is critical to solving this efficiency problem in water splitting. The catalyst design has to be built based on the fundamental understanding of the OER mechanism and the origin of the reaction overpotential. In this article, we summarize the recent progress in understanding OER mechanisms, which include the conventional adsorbate evolution mechanism (AEM) and lattice-oxygen-mediated mechanism (LOM) from both theoretical and experimental aspects. We start with the discussion on the AEM and its linked scaling relations among various reaction intermediates. The strategies to reduce overpotential based on the AEM and its derived descriptors are then introduced. To further reduce the OER overpotential, it is necessary to break the scaling relation of HOO* and HO* intermediates in conventional AEM to go beyond the activity limitation of the volcano relationship. Strategies such as stabilization of HOO*, proton acceptor functionality, and switching the OER pathway to LOM are discussed. The remaining questions on the OER and related perspectives are also presented at the end.

Short O-O separation in layered oxide Na0.67CoO2 enables an ultrafast oxygen evolution reaction

The layered oxide Na_0.67CoO₂ with Na⁺ occupying trigonal prismatic sites between CoO₂ layers exhibits a remarkably high room temperature oxygen evolution reaction (OER) activity in alkaline solution. The high activity is attributed to an unusually short O-O separation that favors formation of peroxide ions by O^--O^- interactions followed by O₂ evolution in preference to the conventional route through surface O-OH^- species. The dependence of the onset potential on the pH of the alkaline solution was found to be consistent with the loss of H⁺ ions from the surface oxygen to provide surface O^- that may either be attacked by solution OH^- or react with another O^-; a short O-O separation favors the latter route. The role of a strong hybridization of the O-2p and low-spin Co^III/Co^IV π-bonding d states is also important; the OER on other Co^III/Co^IV oxides is compared with that on Na_0.67CoO₂ as well as that on IrO₂.

Hollow-Structured Metal Oxides as Oxygen-Related Catalysts

Metal oxide hollow structures with large surface area, low density, and high loading capacity have received great attention for energy-related applications. Acting as oxygen-related catalysts, hollow-structured transition metal oxides offer low overpotential, fast reaction rate, and excellent stability. Herein, recent progress in the oxygen-related catalysis (e.g., oxygen evolution reaction (OER), oxygen reduction reaction (ORR), and metal-air batteries) of hollow-structured transition metal oxides is discussed. Through a comprehensive outline of hollow-structured spinels, perovskites, rutiles, etc., a rational design strategy is provided for an enhanced oxygen-related catalysis performance from the viewpoint of crystal structures. Urgent challenges and further research directions are presented for hollow-structured transition metal oxides toward excellent oxygen-related catalysis.

Recent Advances in Oxygen Electrocatalysts Based on Perovskite Oxides

Electrochemical oxygen reduction and oxygen evolution are two key processes that limit the efficiency of important energy conversion devices such as metal–air battery and electrolysis. Perovskite oxides are receiving discernable attention as potential bifunctional oxygen electrocatalysts to replace precious metals because of their low cost, good activity, and versatility. In this review, we provide a brief summary on the fundamentals of perovskite oxygen electrocatalysts and a detailed discussion on emerging high-performance oxygen electrocatalysts based on perovskite, which include perovskite with a controlled composition, perovskite with high surface area, and perovskite composites. Challenges and outlooks in the further development of perovskite oxygen electrocatalysts are also presented.

Recommended Practices and Benchmark Activity for Hydrogen and Oxygen Electrocatalysis in Water Splitting and Fuel Cells

Electrochemical energy storage by making H₂ an energy carrier from water splitting relies on four elementary reactions, i.e., the hydrogen evolution reaction (HER), hydrogen oxidation reaction (HOR), oxygen evolution reaction (OER), and oxygen reduction reaction (ORR). Herein, the central objective is to recommend systematic protocols for activity measurements of these four reactions and benchmark activities for comparison, which is critical to facilitate the research and development of catalysts with high activity and stability. Details for the electrochemical cell setup, measurements, and data analysis used to quantify the kinetics of the HER, HOR, OER, and ORR in acidic and basic solutions are provided, and examples of state-of-the-art specific and mass activity of catalysts to date are given. First, the experimental setup is discussed to provide common guidelines for these reactions, including the cell design, reference electrode selection, counter electrode concerns, and working electrode preparation. Second, experimental protocols, including data collection and processing such as ohmic- and background-correction and catalyst surface area estimation, and practice for testing and comparing different classes of catalysts are recommended. Lastly, the specific and mass activity activities of some state-of-the-art catalysts are benchmarked to facilitate the comparison of catalyst activity for these four reactions across different laboratories.